For the purpose of increasing the output torque of an internal combustion engine, it is desirable to increase the intake efficiency (volumetric efficiency) which is given by dividing the amount of intake air (mixture or air) which is drawn into the combustion chamber during the downward stroke of the piston by the volume of the cylinder. In the field of internal combustion engines, it has been practiced to experimentally determine the volume of a surge tank and/or the length of each independent intake passage of the intake system to maximize the effect of inertia supercharging that makes use of the inertia of the intake flow. It is also known to provide a resonance chamber and provide a supercharging effect and a damping of intake noises by tuning the resonance frequency of the resonance chamber to the pressure pulsation of the intake air caused by the reciprocating action of the engine. The resonance chamber is connected to the intake passage via a communication passage and the length of this communication passage and other factors determine the operating range (engine rotational speed range) over which the desired resonance effect can be obtained.
Such an operating range over which the desired resonance effect can be obtained can be expanded by allowing the length of the communication passage connecting the resonance chamber with the intake passage to be varied depending on the operating condition of the engine. For instance, the intake system may comprise a plurality of sets of intake passages having different lengths and valves for selecting a particular set of intake passages so that the set of intake passages that match the particular operating condition of the engine may be selected by suitably operating the valves. Such an example is disclosed in Japanese patent laid open publication No. 9-264143. It is also known to provide a resonance chamber that can be moved relative to the intake passage and to form a communication passage with a pair of tubes that are telescopically disposed relative to each other so that the length of the communication passage may be varied by moving the resonance chamber in dependence on the rotational speed of the engine (Japanese UM publication No. 03-29581).
However, according to the first example of the prior art, because the configuration of the intake system can be changed only in a stepwise fashion for maintaining the resonance effect in relation with the changing operating condition, it is not suitable for use as an intake system for an automotive engine whose operating condition changes continually. The second example of the prior art allows the configuration of the intake system to be changed in a continuous manner so as to maintain the resonance effect over a wide range without any substantial break. However, the need for the mechanism for moving the resonance chamber creates various problems. First of all, because the resonance chamber has a significant volume, moving the resonance chamber in dependence on the rotational speed of the engine causes a significant change in the outer profile of the intake system, and this makes it highly difficult to install the engine in a limited space of the engine room of an automobile. Also, the casing for the resonance chamber is inevitably heavy in weight owing to the size requirement of the resonance chamber, and the mechanism for moving the resonance chamber casing is required to be correspondingly powerful. This results in a relatively massive actuator and a large consumption of energy. Also, the large mass of the resonance chamber casing prevents a prompt movement of the resonance chamber casing and this impairs the responsiveness and controllability of the intake system.